1. Field of the Invention
The present invention relates generally to organic waste disposal and, more particularly, to improved ways and means for incinerating sewage sludge.
2. State of the Art
Conventional multiple hearth furnaces include a plurality of hearths superposed in vertically-spaced relationship. When organic waste material is incinerated in such a furnace it is introduced onto the uppermost hearth and is moved by rabble arms from that hearth to the next lower hearth, and so forth so that the waste material travels downwardly in a serpentine path through the furnace. The incinerated waste is removed from the lowermost hearth and disposed of. It is conventional practice to incinerate the waste by burning it with fuel such as natural gas or oil with the addition of air.
The waste often contains combustible organic materials which are volatile at the temperatures in the furnace and which are driven from the waste by heat. When these gaseous organic materials burn, the temperature in the furnace can tend to rise, and it is necessary to maintain the temperature below a predetermined maximum to avoid damage to the furnace. It is conventional practice to control the temperature by introducing large quantities of air into the furnace to cool it when the temperature exceeds a predetermined level.
U.S. Pat. No. 4,013,023 describes an improved system and method for incinerating organic waste material including sewage sludge and similar materials. This patented system and method include introduction of air and fuel such as natural gas or oil but does not limit the temperature in the furnace by the addition of excess air. Rather, air is controllably reduced below the stoichiometric requirement for complete combustion, and thereby temperature is maintained at the desired level. This process results in the production of volatile, combustible gases some of which are burned in the furnace, thereby heating the sewage sludge. The remainder of the combustible gas rises from the furnace and is burned outside the furnace, for example in an afterburner connected to a waste heat recovery boiler to generate useful steam. Alternatively, the combustible gases could be burned in an afterburner intergral to the top of the furnace.
Organic waste and like materials contain volatile materials and fixed carbon, both of which are combustible, and also ash. In the practice of the patented invention it has been found that because the air in the furnace is maintained below the stoichiometric requirement for complete combustion, the fixed carbon is not always substantially burned and is discharged from the furnace. It is known that fixed carbon is combustible in the presence of sufficient oxygen and heat, and therefore loss of it from the furnace represents a loss in heating value.